The present invention concerns a procedure for the fabrication in a shell mold of a cast monocrystalline blade with a cubic structure comprised of one, or of several cylindrical airfoil forms enclosed by at least two symmetrical shoulders perpendicular to the generatrices of the airfoil form, the procedure being of the type in accordance with which the molten metal is poured into the top of the mold. Controlled solidification, which progresses vertically from the bottom up, takes place. A single grain of the crystal is selected by a selection device placed in the lower part of the mold, and at the outlet of which there will be found a single grain with a predetermined orientation and with a [001] direction that merges with the vertical.
There are known procedures for the fabrication of monocrystalline parts, in particular as shown in French patents 1,481,366 and 2,037187. It is known that the formation of monocrystals supposes that several conditions have been met: obtaining of controlled solidification, and selection of a single grain in particular.
Controlled solidification consists in causing the growth of "nuclei" in a molten metal bath in a given direction while at the same time avoiding the appearance of new nuclei by controlling the thermal gradient and the solidification rate. This is brought about by various devices, such as rows of burners with controlled extinction for heating purposes, a screen with a high thermal gradient extending along the mold, or other devices. The part thus obtained has a columnar structure that gives it favorable mechanical strength. Means making it possible to control grain progression, and the rate at which the front solidifies, have been the subject of numerous-inventions, and, in particular, of French Patent No. 78 19567.
Selection of a single grain is obtained in a known fashion by favoring the growth of grains that are favorably oriented at the expense of others. A grain selection device with a helicoidal baffle that makes it possible to obtain a single grain at the outlet from the baffle is described in the article titled "Single Crystal Technology Use Starting", that appeared in Aviation Week, 1 October 1979, pages 69 to 73, but this device does not ensure control of secondary orientation.
When one applies the conventional procedure of the type above to the fabrication of wide chord fixed blades for a turbine nozzle group comprised of a hollow airfoil form between two shoulders, one disposes the axes of these blades vertically in shell molds in order to avoid the joints of transverse grains.
The parts are clustered, but the importance of the shoulders limits considerably the number of blades per cluster.
On the other hand, an examination of large chord blades obtained in accordance With this method shows that the shoulders present an equi-axis structure in the transition zones located at their extremities. In point of fact, because the shoulders are placed perpendicular to the gradient, this placement results in the germination, in the shoulder, of grains that grow with the thickness. The shoulders, upon solidification, have numerous micropipes. These micropipes are due to the fact that by the end of the pouring process the molten metal reaching the ends of the shoulders is unable to force its way between the secondary dendrites to feed solidification shrinkage.